Bluetooth: Host: Tests: Move testlib to common

This is the first patch with the goal of establishing a common library
for Bluetooth utilities that are commonly used for setup in Bluetooth
Host tests. The ultimate goal is to remove redundant (near) copies of
these utilities and other more ad-hoc solutions.

This patch moves one instance of testlib (from
tests/bsim/bluetooth/host/att/long_read) to
tests/bluetooth/common/testlib and makes it a proper CMake library.

The long_read test is updated to link to the new CMake library.

Further changes and de-duplication will come in later patches.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>